The present invention relates to a process for producing a saturated cyclic ether. Particularly, it relates to a process for producing a saturated cyclic ether such as tetrahydrofuran, which comprises reacting a mono- and/or di-fatty acid ester of an xcex1, xcex4-diol in the presence of a solid acid catalyst.
A saturated cyclic ether such as tetrahydrofuran is a compound which is extremely useful as an organic solvent, or as a raw material for e.g. polytetramethylene ether glycol.
As a method for producing tetrahydrofuran, a method of hydrogenating butynediol made from acetylene and formaldehyde to convert it to butanediol, followed by dehydration cyclization, or a method of reacting an acetic acid ester of 1,4-butanediol with water in the presence of an acid catalyst, has, for example, been known.
As the acid catalyst, it is known to employ a liquid acid or a solid acid. As a method of using a liquid acid, a method of carrying out the reaction in a liquid phase by using sulfuric acid, is known (JP-A-52-93762, corresponding U.S. Pat. No. 4,105,679). With respect to a method of using a solid acid, as the reaction method, a method wherein the reaction is carried out by liquid/liquid contact of all substrates in a liquid phase (JP-A-52-7909) or a method wherein the reaction is carried out by gas/liquid contact with an ester in a liquid phase and with water in a gas phase (JP-A-52-95655, JP-A-52-95656), is known.
However, such conventional methods have had some problems. Firstly, in the case of a method of using sulfuric acid as the catalyst, a high concentration sulfuric acid is employed, whereby coloration of the reaction liquid tends to be substantial, and separation from the reaction product tends to be difficult. Further, corrosion of the reactor by sulfuric acid is substantial, and the yield is also low.
Also in the case of using a solid acid catalyst, the conventional methods were carried out by a liquid phase reaction by liquid/liquid contact or by a gas liquid mixed reaction by gas/liquid contact. For example, in JP-A-52-95655 or in JP-A-52-95656, a liquid acetic acid ester and steam were reacted to produce a cyclic ether. In these conventional methods, at least an ester as one of substrates is present entirely in a liquid phase in the reaction system, whereby there have been the following problems.
{circle around (1)} Due to clogging of the catalyst, deterioration in the activity will occur.
{circle around (2)} A liquid is adsorbed on the surface of the catalyst, whereby the flow rate control or the contact time control tends to be difficult.
{circle around (3)} The mixing is not uniform, whereby there will be a portion where the concentration of the ester is locally high, thus leading to deterioration in the activity or an increase of a side reaction.
{circle around (4)} The catalyst component will dissolve in a liquid phase carboxylic acid desorbed under a high temperature condition, whereby deterioration in the activity and in the useful life of the catalyst will result.
It is an object of the present invention to solve such problems of the prior art and to provide a process for producing a saturated cyclic ether such as tetrahydrofuran stably in high yield over a long period of time, by reacting a fatty acid ester of an xcex1, xcex4-diol such as 1,4-butanediol in the presence of a solid acid catalyst.
The present inventors have conducted an extensive study under such circumstances and as a result, have found that in the production of a saturated cyclic ether such as tetrahydrofuran by reacting a fatty acid ester of an xcex1, xcex4-diol such as 1,4-butanediol in the presence of a solid acid catalyst, the above-described problems can be all solved by reacting the fatty acid ester in such a state that at least 50 mol %, preferably substantially all of the fatty acid ester as the reaction substrate, is in a vaporized gas phase state, and thus have completed the present invention.
According to the process of the present invention, as compared with the case of the conventional liquid phase reaction or the gas/liquid contact reaction, the reaction can be carried out within a wide temperature range from a low temperature to a high temperature, formation of by-products is little, and the flow rate control and the contact time control are easy. Further, even by a high temperature reaction or a reaction for a long time, deterioration of the catalyst is less, and even if the amount of water to be supplied, is reduced, the reaction proceeds completely, and the load in the subsequent separation of water can be reduced. Further, deterioration of the catalyst due to a carboxylic acid formed during the reaction will be low as compared with the gas/liquid contact reaction or the liquid phase reaction. Further, as compared with the gas/liquid contact reaction, in the gas phase reaction, the fatty acid ester and water are uniformly dispersed, whereby the reaction efficiency is good, and the yield will be high, and formation of by-products can be suppressed.
Namely, the gist of the present invention resides in a process for producing a saturated cyclic ether, which comprises reacting a mono- and/or di-fatty acid ester of an xcex1, xcex4-diol in the presence of a solid acid catalyst, wherein the reaction is carried out in such a state that at least 50 mol % of the fatty acid ester supplied to the reaction system is vaporized.
Now, the present invention will be described in detail.
In the present invention, the condition that the reaction is carried out in such a state that at least 50 mol % of the fatty acid ester as the reaction substrate supplied to the reaction system, is vaporized, will vary variously depending upon the reaction substrate, the reaction temperature, the reaction pressure and the amount of the inert substance added. However, it is necessary to satisfy the following two points (1) and (2).
(1) In a case where all of the reaction substrates introduced, are gas, the reaction is carried out under such a condition that the molar ratio of raw material components to be introduced into the reaction system, and the saturated vapor pressure of the fatty acid ester at the reaction temperature under the reaction pressure, satisfy the following relation:
(mols corresponding to x mol % of the fatty acid ester/total mols of all materials) less than (saturated vapor pressure of the fatty acid ester at the reaction temperature under the reaction pressure/reaction pressure)
where all materials mean the fatty acid ester, water and gaseous inert components, and x represents the vaporized ratio (mol %) of the fatty acid ester supplied to the reaction system.
In the present invention, the vaporized ratio x is 50 mol %.
(2) In a case where the reaction substrates introduced are liquid even partly, it is necessary not only to satisfy the requirement of the above (1), but also to apply, in addition to the calory required to bring all water introduced, to the reaction temperature and to vaporize it, the calory required to bring the ester introduced, to the reaction temperature and to vaporize at least x mol % thereof, after the introduction of the substrates and before the substrates contact the catalyst.
With respect to the above (1) and (2), in the present invention, the vaporized ratio x is at least 50 mol %.
Here, the calory required for vaporization is determined by considering the method for heating the reactor and conditions such as the thermal conductivity of the material of the reactor, the supplied amount of the calory (per unit time), and the space, volume, etc. until the reaction substrates will contact the catalyst in the reactor.
In the present invention, by adding a gaseous inert component to the reaction system, it is possible to reduce the partial pressure of the fatty acid ester and to facilitate vaporization of the reaction substrate within wider reaction temperature and reaction pressure ranges. Such an inert component is not particularly limited. For example, nitrogen, carbon dioxide, argon, air or oxygen may be employed, and preferably, nitrogen, carbon dioxide or air may be employed.
As an example, a case may be mentioned in which 1,4-diacetoxybutane (hereinafter referred to as DAB) and water are used as reaction substrates, and the reaction is carried out under normal pressure to form tetrahydrofuran. In such a case, the boiling point of DAB is highest at 223xc2x0 C. among the reaction substrates. Accordingly, if the reaction temperature is higher than 223xc2x0 C., all substrates (DAB, water, tetrahydrofuran, acetic acid) will be in a gas phase state, and therefore, there will be no problem. Further, in a case where the reaction temperature is 120xc2x0 C., and the reaction substrates are introduced at 120xc2x0 C. (water is gas, and DAB is liquid), as the saturated vapor pressure of DAB is about 20 mmHg, if the molar ratio of the raw materials introduced is DAB/all materials less than 1/20 (where 1/20 is determined by (20/760) mmHg÷0.5, and all materials mean DAB+water+gaseous inert components), at least 50 mol % can be vaporized, and thus it is simply required to satisfy this condition, and to apply a calory required to vaporize at least 50 mol % of DAB as a reaction substrate, to the reaction system after introducing the substrate.
Likewise, in a case where the reaction temperature is 180xc2x0 C., and the reaction substrates are introduced at 180xc2x0 C. (water is gas, DAB is liquid), and for example, at least 50 mol % of DAB as a reaction substrate is to be vaporized, as the saturation vapor pressure of DAB at 180xc2x0 C. is about 200 mmHg, if the molar ratio of the raw materials introduced is DAB/all materials less than 1/2 (where 1/2 is determined by (200/760) mmHg÷0.5, and all materials mean DAB+water+gaseous inert components), at least 50 mol % can be vaporized, and it is simply required to satisfy this condition, and to apply a calory required to vaporize at least 50 mol % of DAB as a reaction substrate, from the introduction of the substrate and before the substrate is in contact with the catalyst.
In a case where the substrates, the reaction temperature, the reaction pressure and the amount of the gaseous inert component to be added, are different, the conditions may be defined in accordance with the above cases, and it is possible to adjust the vaporized ratio x (mol %) of the fatty acid ester by changing the conditions within wide ranges. Further, with respect to the fatty acid ester as a reaction substrate, at least 50 mol % of the fatty acid ester supplied to the reaction system may be vaporized, and there is no restriction as to the temperature at the time of its introduction. Further, all or a part of the fatty acid ester may be introduced as a gas, or a part or all of the fatty acid ester may be introduced as a liquid and heated in a reaction tube or a pre-heater, to make at least 50 mol % of the fatty acid ester to be a gas.
When at least 50 mol % of the ester is in a gas phase state, the contribution of the gas phase reaction tends to be larger, whereby the present invention will be effective. The larger the proportion (x mol %) of the gas phase state of the raw material ester supplied, the larger the effect. It is preferred to carry out the reaction in such a state that at least 60 mol %, more preferably at least 70 mol %, further preferably at least 80 mol %, particularly preferably substantially all i.e. at least 90 mol % of the fatty acid ester, is vaporized. Most preferably 100 mol % of the raw material ester is in a gas phase state. Accordingly, also with respect to the above-mentioned requirement (1) to accomplish the present invention, it is possible to select the conditions so that the vaporized ratio (x) of the ester will be such a preferred ratio.
Further, in the present invention, it is preferred to supply the fatty acid ester preliminarily vaporized in an amount of at least 50 mol %, and it is more preferred to supply it to the reaction system in such a state that at least 60 mol %, more preferably at least 70 mol % particularly preferably at least 80 mol %, most preferably substantially all i.e. at least 90% of the fatty acid ester, is vaporized.
The xcex1, xcex4-diol in the present invention may be any compound so long as it is a compound having two carbon atoms between two carbon atoms having hydroxyl groups substituted, and it may further have a substituent such as an alkyl group, an alkoxy group or an aryl group. Among them, an alkanediol is preferred, and 1,4-butanediol is particularly preferred.
The mono- and/or di-fatty acid ester of the xcex1, xcex4-diol, to be used in the present invention, is not particularly limited, but it is preferably a diester. Among them, an ester of a saturated fatty acid having from 2 to 4 carbon atoms is preferred, and an acetic acid ester is particularly preferred. Further, the raw material ester may contain 1,4-butanediol or other alcohols or esters, as impurities, so long as they are not compounds adversely affect the reaction and their content is not so large.
In the present invention, a saturated cyclic ether is produced from the mono- and/or di-fatty acid ester of the xcex1, xcex4-diol. Here, the saturated cyclic ether is a compound wherein the ring structure forming the cyclic ether has no unsaturated bond, and the ether ring may further have the above-mentioned substituent such as an alkyl group, an alkoxy group or an aryl group, and may have a substituent having an unsaturated bond. As the saturated cyclic ether, tetrahydrofuran is particularly preferred.
The molar ratio for introduction of raw materials carried out in the present invention may be varied within a wide range so long as the above-mentioned gas phase state of the present invention can be maintained. However, the molar ratio of the raw material ester/all materials is preferably from 0.01 to 1, more preferably from 0.03 to 1, particularly preferably from 0.05 to 1.
As the catalyst to be used in the present invention, a solid acid catalyst showing either a Bronsted acidity or Lewis acidity in the reaction system can be used. Specifically, a zeolite having a crystal structure of X, Y or MFI, or a mesoporous body such as MCM-41 or FSM-16 having a one dimensional mesopore tunnel structure or one having a metal atom such as Ti, Al or Zr incorporated in such a structure, a metal oxide such as silica, alumina, titania or zirconia, a compound oxide having a plurality of metal oxides combined, such as silica/titania, silica/alumina or alumina/titania, and further, a supported type acid catalyst obtained by having a compound with an acidic nature such as H2SO4 or heteropoly acid supported on such an oxide or activated carbon as a carrier, may, for example, be mentioned. Further, it is possible to use a cation exchange resin having acidic functional groups or a solid Bronsted acid such as naphione. Among them, a single metal oxide such as alumina, zirconia or titania, a hydrous metal oxide such as niobic acid, or a mixed oxide such as titania/silica or zirconia/silica, is preferred, and particularly preferred is zirconia or niobic acid.
The reaction of the present invention can be carried out by any of a batch process, a semi-batch process or a continuous process in the same manner as a conventional method, but it is advantageous to carry out the reaction by a continuous system.
The supply velocity (LHSV) of the fatty acid ester of 1,4-butanediol per unit volume of the catalyst, can be changed within a wide range, but it is usually from 0.01 to 1,000 hrxe2x88x921, preferably from 0.05 to 500 hrxe2x88x921, more preferably from 0.1 to 100 hrxe2x88x921. The amount of water to be used in this reaction is not particularly limited, but it is usually from 0.01 to 100 times, preferably from 0.1 to 50 times, more preferably from 0.3 to 20 times, particularly preferably from 0.5 to 15 times, by a molar ratio, to the fatty acid ester of 1,4-butanediol.
According to the present invention, as compared with a conventional catalyst, even in a case of a small amount of water (in a case where a difatty acid ester of an xcex1, xcex4-diol is used as the raw material, an amount of 1 time by mol to the minimum amount of the diacetic acid ester required for complete reaction), specifically in a case of at most 5 times by mol, preferably at most 3 times by mol, to the amount of the fatty acid ester supplied, tetrahydrofuran can be produced easily at a high conversion and with high selectivity. Further, water remaining after the reaction can thereby be minimized, and the load for the subsequent separation between the product and water, can be reduced. Further, in a case where a fatty acid ester of the xcex1, xcex4-diol as the raw material is entirely monoester, the reaction can be proceeded completely without using water. Further, in a case where the fatty acid ester of 1,4-butanediol is a mixture of a mono-ester and a di-ester, the minimum amount of water required to let the reaction proceed completely, is an amount of 1 time by mol to the contained diester. In the present invention, when the reaction is carried out in the presence of water, it is preferred to carry out the reaction in such a state that the fatty acid ester as a reaction substrate, and water are substantially entirely vaporized.
The water to be used in the present invention is not particularly limited, and water separated and recovered after the reaction may be re-used. In the case of such re-use, it is possible to use water containing a small amount of a fatty acid such as acetic acid, an alcohol such as 1,4-butanediol or an ester such as the raw material ester, so long as it is not a compound which substantially adversely affect the reaction.
The temperature for the reaction in the present invention may be within a wide range, but it is usually from 180 to 350xc2x0 C., preferably from 190 to 330xc2x0 C., more preferably from 200 to 300xc2x0 C. Even in a high temperature reaction, according to the process of the present invention wherein reaction is carried out in such a state that at least 50 mol % of the ester as a reaction substrate is in a gas phase state, there is no substantial increase of by-products or no substantial deterioration of the catalyst, whereby the reaction can be carried out at a high temperature which is advantageous from the viewpoint of the reaction rate. In the present invention, it is more preferred to carry out the reaction at a reaction temperature higher than the boiling point of the fatty acid ester under the pressure of the actual reaction.
The pressure in the present reaction is not particularly limited, but it is usually within a range of from 0.01 to 1 MPa, preferably from 0.03 to 0.8 MPa, more preferably from 0.05 to 0.5 MPa.
With respect to the saturated cyclic ether such as crude tetrahydrofuran thus obtained, in a case where the raw material is 1,4-diacetoxybutane, 1,4-diacetoxybutane as the unreacted raw material, acetic acid, water, etc., may be separated and purified by distillation. The separated raw material such as 1,4-diacetoxybutane, may be recycled to the reaction system.